This invention relates, in general, to combined cycle power plants and, in particular, to an improvement in cycle arrangement for optimizing power plant efficiency and providing adaptability to gas turbine fuels having varying sulfur content.
A combined cycle power plant utilizes a gas turbine and a steam turbine in combination to output power, typically electric power. The power plant is arranged so that the gas turbine is thermally connected to the steam turbine by means of a heat recovery steam generator (HRSG). The HRSG is a noncontact heat exchanger which allows feedwater for the steam generation process to be heated by otherwise wasted gas turbine exhaust gases. The HRSG is a large stack with tube bundles interposed therein whereby water is heated to steam as exhaust gases are passed through the stack. The primary efficiency of the combined cycle arrangement is, of course, due to the utilization of otherwise wasted gas turbine exhaust gases.
One key parameter in optimizing the combined cycle efficiency is that the highest efficiency is achieved with the lowest stack gas temperature at the outlet end of the exhaust gas stack. The lower limit on stack gas temperature is usually proscribed by the sulfur content in the gas turbine fuel. This is because sulfur compounds condense on the tube bundles at certain relatively low temperatures causing severe corrosion on the tube bundles. It is also known that the dew point of the corrosive sulfur compounds increases with increased concentration of sulfur in the fuel.
The conventional method for optimizing a combined cycle plant efficiency is to design the HRSG and steam system to operate with a stack gas temperature that would prevent low temperature heat transfer surface corrosion commensurate with the highest level of sulfur content in the fuel expected to be burned in the specific application. If fuel is burned with lower fuel sulfur content, the HRSG stack gas temperature cannot be lowered to improve efficiency although the sulfur compound concentration would allow it. Conversely, if the HRSG were designed with stack gas temperature commensurate with the lowest fuel sulfur content to be expected, the plant efficiency would be improved; however, the HRSG heat transfer surface would experience corrosion if the fuel with higher sulfur content were burned.
The HRSG includes a plurality of interconnected tube bundles which may be identified from top to bottom as a low pressure economizer, a high pressure economizer, an evaporator and a superheater. The HRSG heat exchange process is a counterflow process in that the temperature of the hot exhaust gases decreases as they rise in the stack whereas the temperature of the steam water mixture in the tubes increases as it descends downwardly against the upward flow of hot exhaust gases.